Arun Pal

The Triplet–Singlet Gap in the m-Xylylene Radical: A Not So Simple One

Meta-benzoquinodimethane (MBQDM) or m-xylylene provides a model for larger organic diradicals, the triplet-singlet gap being the key property. In the present work this energy difference has been the object of a systematic study by means of several density functional theory-based methods including B3LYP, M06, M06-2X, HSE and LC-ωPBE potentials and a variety of wave function-based methods such as complete active space self consistent field (CASSCF), Multireference second-order Møller-Plesset (MRMP), difference dedicated configuration interaction (DDCI), and Multireference configuration interaction (MRCI). In each case various basis sets of increasing quality have been explored, and the effect of the molecular geometry is also analyzed. The use of the triplet and broken symmetry (BS) solutions for the corresponding optimized geometries obtained from B3LYP and especially M06-2X functionals provide the value of the adiabatic triplet-singlet gap closer to experiment when compared to the reported value of Wenthold, Kim, and Lineberger, (J. Am. Chem. Soc. 1997, 119, 1354) and also for the electron affinity. The agreement further improves using the full π-valence CASSCF(8,8) optimized geometry as an attempt to correct for the spin contamination effects on the geometry of the BS state. The CASSCF, MRMP, and MRCI, even with the full π valence CAS(8,8) as reference and relatively large basis set, systematically overestimate the experimental value indicating either that an accurate description must go beyond this level of theory, including σ electrons and higher order polarization functions, or perhaps that the measured value is affected by the experimental conditions.

On the Photomagnetism of Nitronyl Nitroxide, Imino Nitroxide, and Verdazyl-Substituted Azobenzene

The cis- and trans-azobenzenes are known as photochromic isomers with the trans- converting into the cis-form and vice versa upon irradiation with specific wavelengths. We have quantum chemically investigated the cis- and trans-forms of substituted azobenzene diradicals, with two nitronyl nitroxides, imino nitoxides, or verdazyls at para positions and serving as monoradical centers, to determine whether they can exhibit a photoassisted magnetic crossover. Geometries of both substituted and unsubstituted molecules have been optimized by density functional (DF) method UB3LYP using the 6-311G(d,p) basis set. Optimization of the geometry of the cis isomers has required special care. Single point singlet, triplet, and broken symmetry calculations have been done using 6-311++G(3df, 3pd) basis set. The magnetic exchange coupling constants have been estimated from the broken symmetry calculations. Absorption wavelengths have been estimated for both substituted and unsubstituted species from time-dependent DF treatment using restricted spin-polarized methodology RB3LYP and 6-311++G(3df, 3pd) basis set. From the similarity in the calculated absorption wavelengths for the unsubstituted and substituted azobenzenes, and the increased oscillator strengths (f) for the substituted species, we predict that the diradical isomers would be strongly photochromic. From our triplet state and broken symmetry calculations, we predict that both the cis- and the trans-diradicals are antiferromagnetically coupled. This prediction is consistent with the spin alternation rule, and the possibility of a magnetic crossover is nonexistent for these species.

Structural variation of transition metal coordination polymers based on bent carboxylate and flexible spacer ligand: polymorphism, gas adsorption and SC-SC transmetallation

Reaction of the bent dicarboxylate ligand H2OBA (H2OBA = 4,4′-oxybisbenzoic acid) and the flexible linker 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (L1), under diverse reaction conditions, forms two polymorphic Co(II) coordination polymers (CPs): {[Co(OBA)(L1)]·DMF}n (1), as a three dimensional (3D) framework with a pcu alpha-Po primitive cubic topology, and {[Co(OBA)(L1)]·DMF}n, (2), as a two dimensional (2D) structure with a 6-c uninodal net topology. Gas adsorption measurements of the desolvated Co(II) CPs show negligible uptake of all gases in 1, while 2 exhibits moderate uptake of CO2, with good selectivity over N2 and CH4. With Zn(II), reaction of H2OBA and L1 produces a different 2D CP, {[Zn0.5(OBA)0.5(L1)0.5]}n (3). Finally, three isostructural Cd(II) CPs, {[Cd(OBA)(L1)]·DMF}n (4), {[Cd(OBA)(L1)]·DEF}n (5), and {[Cd(OBA)(L1)]·DMA}n (6) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, DMA = N,N-dimethylacetamide), that differ only in the lattice solvent molecules and show 2D structural arrangements are prepared. Interestingly, CP 4 undergoes single-crystal to single-crystal (SC-SC) transmetallation reaction at room temperature, yielding isostructural {[Cu(OBA)(L1)]·DMF}n (7) that cannot be synthesized independently. Moreover, the luminescence properties of compounds 1, 2, 3, and 4 have been studied in the solid state at room temperature. All the complexes are characterized by elemental analysis, IR, TGA, PXRD and single crystal X-ray diffraction.

Quantum Chemical Investigation of meta-Xylylene Based One-Dimensional Polymer Chain

We have investigated unsubstituted and methyl substituted polyradical chains of meta-xylylene by using density functional theory-broken symmetry methodology (DFT-BS). Optimization of geometry in the high-spin and low-spin states have been done at B3LYP/6-31G(d,p) and M06-2X/6-31G(d,p) levels in unrestricted methodology. Single-point calculations on the high-spin optimized geometries have been done by using the 6-311G(d,p) basis set. Each polyradical has been found to be nonplanar with a high-spin ground state. Each has a coupling constant larger than thermal energy. For each group of polyradicals, the coupling constant has been found to exponentially decrease with increase in the number of phenylene groups. The B3LYP infinite chain limit has been estimated for both the unsubstituted and substituted polyradicals. The individual inter-radical-site coupling constants have been estimated for the triradicals and tetraradicals using HDVV Hamiltonian in ORCA 3.0.1 code. These are also generally large and positive, revealing a strong intersite ferromagnetic interaction. The intersite coupling constant too decreases with increasing distance between the radical centers. Finally, we have used CRYSTAL09 package for calculations on the infinitely long one-dimensional and periodic polyradical chains. The coupling constants estimated from the periodic calculations are quite large at about 500 cm(-1) and somewhat greater than the limiting values calculated for the polyradicals with an increasing number of phenylene groups. This happens as the individual polyradicals of increasing size progressively deviate from periodicity, and thereby lessens the strength of through-bond spin-spin coupling. The calculated band gap of ∼4.5 eV indicates that the infinitely long one-dimensional chains must be ferromagnetic and electron insulators.

Theoretical investigation of photomagnetic properties of oxoverdazyl-substituted pyrenes.

We have investigated the ground state spin of 10 pairs of possible photochromic diradical isomers by quantum chemical methods. Dihydrogen pyrenes and dinitrile pyrenes have been chosen as spacers with radical centers attached at (1,7) and (1,8) locations. Oxoverdazyl has served as a radical center, and both C and N linkages have been investigated. Triplet molecular geometries have been optimized at the UB3LYP/6-311G(d,p) level. Single-point calculations on triplet and broken symmetry states have been performed using the 6-311++G(d,p) basis set. Careful designs have led to the prediction of strongly coupled dihydropyrene (DHP) isomers, and the cyclophenadiene (CPD) isomers have always been found as weakly coupled. The effect of the functional M06-2X has been investigated. Calculated TDDFT spectra have been sufficient to guarantee photochromism of the designed diradicals. It has been estimated that compounds of diradicals with large coupling constants in the DHP form would show a pronounced change in molar susceptibility on photoconversion. This has led us to identify two molecules that can serve as a photomagnetic switch at room temperature.

Two azo-functionalized luminescent 3D Cd(II) MOFs for highly selective detection of Fe3+ and Al3+

Two cadmium-based 3D luminescent MOFs {[Cd2(SA)2(L)2]·H2O}n (Cd-MOF-1) and [Cd(CDC)(L)]n (Cd-MOF-2) (H2SA = succinic acid, H2CDC = 1,4-cyclohexanedicarboxylic acid, L = [3,3′-azobis(pyridine)]) have been assembled by employing organic dicarboxylic acid linkers with an unexploited azo-functionalized N,N′ spacer via a room temperature slow evaporation process, and they are characterized by single crystal X-ray analysis, TGA, FT-IR, PXRD and elemental analysis. The topological analysis reveals that Cd-MOF-1 features a 6c-uninodal rare ‘rob’ topology with the point symbol {48·66·8}, whereas Cd-MOF-2 shows a ‘pcu’ alpha-Po primitive cubic topology with the point symbol {412·63}. These MOFs are highly emissive at 382 nm and 398 nm when excited at 305 nm and 312 nm, respectively. The exposed azo groups are presumed to act as functional sites for the recognition of metal ions through quenching of fluorescence intensity. The fluorescence measurements show that these MOFs can selectively and sensitively detect Fe3+ as well as Al3+ and thus, they demonstrate potential as dual-responsive luminescent probes for metal-ion sensing. EDS elemental mapping, PXRD of loaded MOF materials, and a plausible quenching mechanism have been discussed. More importantly, both the MOFs exhibit rapid response times toward sensing of Fe3+ and Al3+.